Apparatus and method for summing threshold inputs in a coincidence event detector

ABSTRACT

An apparatus and method for reducing the non-uniformity of a positron emission tomography (“PET”) image is described. The event detector system comprises a detector having a plurality of zones, each zone comprising a plurality of detector devices. The event detector system also comprises a threshold circuit coupled to a detector device of the detector. Additionally, the event detector system comprises a summation circuit coupled to the threshold circuit.

FIELD OF THE INVENTION

The present invention relates to generating images using a coincidence event detector. More particularly, the present invention relates to normalizing the triggering events in a coincidence event detector, thus reducing image artifacts in the generated image.

BACKGROUND OF THE INVENTION

Many types of equipment are designed to detect physical events such as particle-matter interactions. In the prior art, event detection has been widely used in scientific research and medicine to generate images of a patient's internal structures. For example, nuclear medicine cameras (also referred to as a gamma cameras) may perform positron emission tomography (“PET”) on emitted photon pairs to map the originating location and energy of the emitted photon pairs, thus allowing the nuclear medicine cameras to generate images of tumors or other cancerous developments.

Typically, to generate the image, a patient is injected with a radio pharmaceutical (such as Flouro Deoxi Glucose) that interacts with electrons in the body of the patient. As a result of the interaction, positrons are annihilated and gamma rays, including photon pairs, are emitted from the body of the patient in directions of travel that are 180° apart from each other. An example of a prior art PET may be found in the dual single-photon completed emission tomography (“SPECT”)/PET system of U.S. Pat. No. 5,608,221 of Bertelsen et. al (issued Mar. 4, 1997 and assigned to ADAC Laboratories of Milpitas, Calif.).

Certain nuclear medicine camera systems include two planar detector assemblies that are positioned on opposite sides of the patient. Each detector assembly includes a crystal covered by an array of photo multiplier tubes (“PMTs”). To perform the PET, nuclear medicine cameras detect the collision of photons with the detector assemblies. Specifically, when a photon comes in contact with the crystal of a detector assembly, a scintillation occurs. In response to the scintillation, the PMTs exposed to the scintillation event generate electrical outputs. Accordingly, the electrical output of the PMTs may be used to detect the emission of a coincidence photon pair—hereinafter referred to as a coincidence event.

As previously described, certain nuclear medicine camera systems include two planar detector assemblies. Accordingly, to detect a coincidence event nuclear medicine cameras use a PET coincidence detection system. In particular, in a PET coincidence detection system, the PMTs of a first detector assembly are divided into spatial quadrants (or zones) and the electrical outputs of the zones are summed. The summated outputs are digitized and logically combined with the digitized outputs of all the zones in the first detector assembly, thus generating a first coincidence trigger. Similarly, the PMT outputs of the second detector are summed and digitized to generate a second coincidence trigger. Accordingly, if the two coincidence triggers transition within a predetermined time window (typically 10-20 nanoseconds), the scintillations causing the transition of the coincidence triggers are deemed to be from a coincidence event. Thus, the location and energy of the scintillations on the PMTs may be used to 5 determine the origin of the event.

FIG. 1 is a block diagram of a prior PET coincidence detection system 100. System 100 includes two detectors 166 and 168. Detector 166 is divided into four zones 166 (1), 166 (2), 166 (3), and 166 (4). Each of zones 166 (1-4) include multiple PMTs. Zone 166 (1) operates similarly to other zones in detector 166 and corresponding zones in detector 168. Accordingly, zone 166 (1) will be described as an example of how a zone of a detector operates. When any PMT in zone 166 (1) detects a scintillation event resulting from a collision of a photon with the crystal of detector 166 (not shown), an analog signal is sent to summing circuit 101. Summing circuit 101 receives signals from all of the PMTs in zone 166 (1) and sums their amplitudes in a known manner for example via a series weighted resistors. Summing circuit 101 outputs a signal to constant fraction discriminator (“CFD”) 131. CFD 131 operates as a trigger detector in an amplitude independent manner. CFD 131 outputs a zone trigger signal to programmable delay 141.

Programmable delay 141 is typically controlled by a processor of system 100 and is used to vary the delay of the trigger signal output by CFD 131 during calibration of system 100. Zones 166 (2), 166 (3), and 166 (4) operate in the same manner as zone 166(1), each outputting a signal from their respective programmable delay circuits indicating that a coincidence event has been detected. The outputs of programmable delays 141, 142, 143, and 144 are input to OR gate 156. Detector trigger signal 170 is active on the output of OR gate 156 when any event is detected in a zone of detector 166. Detector trigger signal 170 is input to common delay CD_(A) 160, which is associated with a detector 166. CD_(A) 160 is a programmable delay circuit that is used to vary the delay of detector trigger signal 170 with respect to detector trigger signal 171. Adjusted detector trigger signals 174 and 176 are input to coincidence detection circuit 164. Coincidence detection circuit 164 typically performs an operation such as an AND operation for determining when scintillation events have been detected simultaneously in detectors 166 and 168.

As illustrated in FIG. 1, system 100 includes multiple delay elements to synchronize the detection of coincidence events. In particular, programmable delays 141-144 and 151-154 are adjusted to compensate for variances such as response time of different PMTs, different cable lengths between zones, and different component tolerances between zones. Additionally, CD_(A) 160 and CD_(B) 161 are adjusted for the purpose of synchronizing the global, or common, delays of signals 170 and 171 due to cable lengths between OR gates 156 and 158 and coincidence detection circuit 164.

Although a PET system using system 100 may generate accurate images based on the detection of events, the coincidence detection system has numerous disadvantages. One disadvantage, results from the non-uniformity of the images generated by the PET. In particular, the low number of zones in system 100 create “dead areas” during the detection of multiple events. For example, if an event causes a scintillation in the PMTs shared by zone 166(1) and zone 166(2), neither zone is available to detect subsequent scintillations until the PMT signals correlating to the event have been processed. Thus, during processing of the event, zone 166(1) and zone 166(2) may not detect a coincidence event.

Another disadvantage results from the prior art techniques used to overcome the non-uniform images generated by PET systems. In particular, prior art PET systems use calibration techniques to overcome the generation of non-uniform images. Typically, the calibration technique involves the characterization of the sensitivity profiles of the coincidence detection system. Subsequently, the characterization profiles are used to reduce the non-uniformity of the PET generated image. The use of calibration techniques, however, tends to result in the loss of image data.

SUMMARY OF THE INVENTION

An apparatus and method for an event detector system are described. The event detector system comprises a detector having a plurality of zones, each zone comprising a plurality of detector devices. For one embodiment, each detector device is operable to generate an event signal in response to an energy source. The event detector system also comprises a threshold circuit coupled to a detector device of the detector. For another embodiment, the threshold circuit is operable to selectively transfer the event signal of the detector device. Additionally, the event detector system comprises a summation circuit coupled to the threshold circuit. For yet another embodiment, the summation circuit is operable to generate a trigger signal in response to the event signal of the detector device.

The method for selecting a threshold value in an event detector comprises determining a detector configuration. The method also comprises applying a first threshold value to a threshold circuit. Additionally, the method comprises generating an energy source. For one embodiment, generating the energy source comprises directing a light source on the event detector. The method further comprises examining the detector configuration to determine the number of triggering channels engaged by the energy source.

Other features and advantages of the present invention will be apparent from the accompanying drawings and from the detailed description that follows.

BRIEF DESCRIPTION OF THE DRAWINGS

The features and advantages of the present invention are illustrated by way of example and not limitation in the figures of the accompanying drawings in which like references indicate similar elements and in which:

FIG. 1 shows a prior art coincidence detection system;

FIG. 2 illustrates one embodiment of a nuclear medicine imaging system;

FIG. 3 illustrates one embodiment of a scintillation causing a light spread function on a detector;

FIG. 4 illustrates one embodiment of the circuitry used to process signals generated by photo multiplier tubes of a detector;

FIG. 5 illustrates one embodiment of a threshold circuit; and

FIG. 6 shows a flowchart illustrating the selection of a threshold value for a threshold circuit.

DETAILED DESCRIPTION

An apparatus and method for reducing the non-uniformity of positron emission tomography (“PET”) images are described. For one embodiment, a PET system comprises an array of photo multiplier tubes (“PMTs”) divided into zones—also referred to as triggering channels. The PET system uses the PMTs to generate electrical signals in response to a scintillation—also referred to as an energy source. For one embodiment, the scintillation is distributed across the PMTs according to a light spread function. Accordingly, the scintillation may activate a large number of triggering channels, thus causing the PET system to generate non-uniform images. In certain embodiments, the PET described system may be incorporated in a dual PET/SPECT system.

Thus, an intended advantage of an embodiment of the invention is to improve the sensitivity profile of a PET detector. Another intended advantage of an embodiment of the invention is to reduce the number of triggering channels engaged for a given scintillation—i.e. reduce the dead area for a given event. Yet another intended advantage of an embodiment of the invention is to provide an adjustable threshold circuit that reduces the non-uniformity of a PET image.

FIG. 2 illustrates one embodiment of a nuclear medicine imaging system. For one embodiment, system 200 comprises two modes of operation, either a single photon emission computed tomography (“SPECT”) mode or a PET mode. The operation mode is determined by mode signal 230 transmitted from acquisition processor 234 to detection circuit 232. For another embodiment, system 200 comprises four triggering channels per detector. For yet another embodiment, system 200 comprises seventeen triggering channels per detector.

System 200 also comprises event detector assembly 202 and event detector assembly 204 arranged 180° apart on opposite sides of patient 240. System 200 also comprises gantry 214 coupled to base 216. For one embodiment, gantry 214 positions detector assemblies 202 and 204 about patient 240 while patient 240 rests on table 242. For another embodiment, event detector assemblies 202 and 204 each include circuitry for generating detector trigger signals on bus lines 222 and 224. Additionally, event detector assembly 202 includes multiple PMTs (not shown) in a planar array coupled to glass plates 206. Similarly, event detector assembly 204 includes multiple PMTs (not shown) in a planar array coupled to glass plates 208. As further illustrated in FIG. 2, glass plates 206 and 208 are coupled to crystals 210 and 212, respectively. For one embodiment, glass plates 206 and 208 comprise pyrex. For another embodiment, crystals 210 and 212 are each single sodium iodide (“NaI”) crystals doped with thallium (“TI”).

For one embodiment, scintillations are produced by photons emitted from patient 240 with crystals 210 and 212. To qualify as a coincidence event, however, a scintillation should occur on crystal 210 within a predetermined time period of a scintillation on crystal 212. For example, for one embodiment, if the first photon of a photon pair collides with crystal 210 and 10 nanoseconds later the second photon of the photon pair collides with crystal 212, a coincidence event has occurred. Thus, to determine a coincidence event detection circuit 232 examines (1) whether a trigger channel has been engaged (i.e. activated) and (2) the time period between the activation of a triggering channel in event detector assembly 202 and event detector assembly 204.

For one embodiment, to determine whether a trigger channel has been activated, system 200 determines the number of PMTs activated in a given triggering channel via bus line 222 or bus line 224. In particular, the bus lines (222 and 224) transmit PMT signals from the event detector assemblies (202 and 204) to detection circuit 232. Thus, when a photon from patient 240 collides with crystal 210, the PMTs of event detector assembly 202 that are exposed to the scintillation generate signals on bus line 222. Similarly, when a photon from patient 240 collides with crystal 212, the PMTs of event detector assembly 204 that are exposed to the scintillation generate signals on bus line 224. The relation between a scintillation and the PMT signals is described below with respect to FIG. 3.

For another embodiment, detection circuit 232 determines the timing relation between the activation of two triggering channels via delay logic and constant fraction discriminators (“CFDs”) (not shown). The circuitry of detection circuit 232 is described below with respect to FIG. 4.

As previously described detection circuit 232 determines that a coincidence event has occurred if a trigger channel has been activated in each of the event detector assemblies (202 and 204) and if both trigger channels were triggered in a given time period. In response to the detection of the coincidence event, valid event trigger signals 218 and 220 are transmitted to event detector assembly 202 and event detector assembly 204, respectively. The valid event trigger signals 218 and 220 are used to trigger integrators (not shown) associated with the pre-amplification and digitization of PMT signals recovered by the event detector assemblies. Subsequently, system 200 begins processing of the coincidence event that created event 305. Specifically, the event detector assemblies (202 and 204) transmit spatial coordinate information to acquisition processor 234 via spatial coordinate signals 226 and 228. Acquisition processor 234 uses spatial coordinate signals 226 and 228 to produce input to image processor 236. For one embodiment, image processor 236 produces an image of tissues in patient 240 showing a growth such as tumors. The image produced by image processor 236 is displayed on display unit 238.

FIG. 3 illustrates one embodiment of a scintillation causing a light spread function on a detector. In particular, system 300 shows PMTs (9-55) coupled to glass plate 306, which in turn, is coupled to crystal 308. System 300 also shows a photon (305) interacting with a crystal (308) and generating light spread function 307. For one embodiment, PMTs 9-55 are divided into three triggering channels. The triggering channels comprise a left channel (PMTs 9, 27, and 42), a middle channel (PMTs 49,51, and 55), and a right channel (PMTs 19, 36, and 48). As illustrated in FIG. 3, the light spread function is distributed across PMTs 42-48. The energy of light spread 307, however, is concentrated in the middle channel. Thus, the PMT signals (i.e. analog signals) generated by the PMTs in the middle channel will have a higher voltage than the PMT signals generated by PMTs in the left or right channel. For alternative embodiments, the PMT signals generated by the PMTs in the middle channel may have a higher current or faster rise time than the PMT signals generated by PMTs in the left or right channel

For one embodiment, system 300 corresponds to event detector assembly 204 of system 200. Accordingly, crystal 308 corresponds to crystal 212 and glass plate 306 corresponds to glass plate 208. Additionally, the outputs of PMTs 9-55 are coupled to detection circuit 232 via bus line 224.

As previously described, the energy of light spread 307 is concentrated in the middle channel. Accordingly, for one embodiment, detection circuit 232 comprises threshold circuitry that activates a triggering channel or channels in the area of concentrated light spread function by selectively transferring PMT signal that exceed a threshold value. The threshold value may be set according to a voltage, a current, or the rise time of a signal. Thus, in the present embodiment, only the middle channel is triggered. The sole triggering of the middle channel results in the left and right channel being available to detect subsequent events. Accordingly, the dead area in system 200 is reduced. Additionally, the non-uniformity of the images generated by system 200 are reduced because a smaller number of triggering channels are used to detect a coincidence event.

FIG. 4 illustrates one embodiment of the circuitry used to process signals generated by photo multiplier tubes of a detector. In particular, system 400 comprises a group of threshold circuits (“TCs”) coupled to coincidence timing circuit 420. For one embodiment, system 400 is used in detection circuit 232 of system 200, thus coincidence timing circuit 420 generates a valid event trigger signal on lines 218 and 220. Additionally, each TC of system 400 is coupled to a PMT of event detector assemblies 202 or 204 via bus lines 222 and 224. For another embodiment, TCs 401-401 n are coupled to the PMTs of a triggering channel on event detector assemblies 202. Additionally, TCs 404-404 n are coupled to the PMTs of event detector assemblies 204. For yet another embodiment, the boundaries of triggering channels may overlap, thus a PMT may be coupled to TC 401 and TC 401 n.

As illustrated in FIG. 4, TCs 401-401 n are coupled to summing circuit 402. For one embodiment, summing circuit 402 adds the amplitudes of the signals generated by TCs 401-401 n. Subsequently, summing circuit 402 outputs a signal to CFD 403 which outputs an event trigger signal independent of the amplitude of the incoming signal. For another embodiment, CFD 403 produces a trigger based upon a constant fraction of an overall pulse rather than an amplitude of the pulse. For example, CFD 403 examines the area of the pulse in the region of 10% to 90% of the output created by summing circuit 402. The event trigger signal output by CFD 403 is then input to programmable delay 405 and a trigger channel event signal is generated on line 407. For one embodiment, programmable delay circuit 405 is programmable to adjust the flight time of signals on line 407, thus ensuring that trigger channel event signals on lines 407 and 417 have a minimum skew.

The trigger channel event signals (407 and 417) are inputs to coincidence timing circuit 420. For one embodiment, coincidence timing circuit comprises latches and logic gates (not shown) that determine the timing difference between transition of the trigger channel event signals. For another embodiment, coincidence timing circuit 420 generates a valid event trigger signal if the difference between transition times of the trigger channel event signals falls within a predetermined time period.

For one embodiment, the threshold circuits (TC 401-404 n) are operable to improve the sensitivity profile of the summation circuits (402 and 412), thus reducing image non-uniformity. In particular, the threshold circuits are operable to create threshold levels so that PMTs of a detector may not engage a triggering channel unless the PMTs produce a signal that surpasses the threshold level. This threshold constraint on the engagement of a triggering channel reduces the dead areas in an imaging system, thus reducing image non-uniformity in PET generated images.

FIG. 5 illustrates one embodiment of a threshold circuit. In particular, threshold circuit 500 comprises three resistors (R510-520), two amplifiers (A530 and A535), a precision diode (540), and a select logic (S545). For one embodiment, S545 comprises a switch that selectively couples line 550 to line 505 or the output of A535. For another embodiment, threshold circuit 500 corresponds to TC 401. Accordingly, line 505 is coupled to a PMT output and line 550 is coupled to summing circuit 402. Additionally, threshold circuit 500 may directly transfer the PMT signal from line 505 to line 550 via S545. Alternatively, threshold circuit 500 may transfer the output of A535 to line 550, thus transferring the PMT signal processed by threshold circuit 500.

For one embodiment, threshold circuit 500 transfers the signal on line 505 to line 500. Accordingly, diode 540, amplifier A530, and amplifier A535 are used to generate a signal on line 505. In particular, A530 comprises two inputs and an output. The first input is coupled to R510, R515, and the output of diode 540. The second input of A530 is coupled to a reference voltage (Vref 525). Similarly, A535 comprises two inputs and an output. The first input is coupled to R520. The second input of A535 is coupled to R515 and the output of A535 is coupled to S545.

The voltage of Vref 525 and the resistive values of R510-520 may be varied to selectively transfer a PMT signal from line 505 to line 550, thus effectively clamping any PMT signals below the threshold value.

For example, for one embodiment, a PMT signal on line 505 ranges in value from 50 millivolts (mv) to 500 mv and threshold circuit is operable to remove PMT signals below 200 mv. Accordingly, V ref has a value of 120 mv. Additionally, in the present embodiment, R510, R515, and R520 have a value of 205 ohms, 100 ohms, and 100 ohms, respectively.

FIG. 6 shows a flowchart illustrating the selection of a threshold value in a threshold circuit. In particular, flowchart 600 comprises blocks 610 through 660. Flowchart 600 may be applied to determine the threshold value for a single threshold circuit. Flowchart 600 may also be applied to determine the threshold value for a group of threshold circuits. For example, for a given triggering channel, flowchart 600 may be used to determine the threshold value for all PMTs located at the edge of the triggering channel.

For one embodiment, blocks 610 through 660 show the steps used to vary the voltage Vref 525 of threshold circuit 500. The voltage of Vref 525 may be varied to ensure that a PMT signal exceeds the threshold value prior to transferring the PMT signal to a summing circuit.

As illustrated in FIG. 6, operation begins in block 610. At block 610, the detector configuration is determined. In particular, for one embodiment, at block 610 the total number of triggering channels in the detector assembly is determined. The total number of triggering channels is used to identify the percentage of triggering channels engaged for a given event. For another embodiment, at block 610, the location of the PMT tubes with respect to the triggering channels are determined. Accordingly, the PMTs that may overlap into multiple triggering channels are identified in block 610. Thus, if flowchart 600 is applied to a PMT used in overlapping triggering channels, the threshold value may be modified so that the PMT triggers a single zone. For yet another embodiment, at block 610, the field of view of a crystal of the event detector is used. Specifically, the field of view of the crystals of a detector are used to define the active region of the detector. After the detector configuration is determined, block 620 is processed.

At block 620, the threshold value is set. In particular, the voltage value of Vref 525 is set to a first value. For one embodiment, the voltage value is determined based on the detector configuration identified in block 610. Specifically, if the threshold circuit is coupled to a PMT located in the middle of a triggering channel the threshold value may be set to a smaller value than if the threshold circuit is coupled to a PMT located at the edge of a triggering channel. After setting the threshold value, block 630 is processed.

At block 630 a simulated light spread function is applied to a pre-determined event location. The pre-determined event location may be place in the middle of a triggering channel or on the edge of a triggering channel. For one embodiment, the location and the intensity of the pre-determined event may be varied for a given field of view. Specifically, for a given field of view, block 630 may be looped while the location or the intensity of the pre-determined event is adjusted. For example, in a field of view measuring 20 centimeters (“cm”) by 40 cm, the event location may be incrementally adjusted by 1 millimeter. Accordingly, the simulated light spread function is repetitively applied to each of the adjusted event location. Thus, block 630 may be used to determine (i.e. map) the number of triggering channels activated by the first threshold value for a given location.

In another example, for a field of view measuring 20 cm by 40 cm, the intensity of the event may start at 100 kiloelectronvolts (“keV”), the unit of measure for a gamma-ray photon energy, and may be incrementally adjusted by 1 keV. Accordingly, the adjusted simulated light spread function is repetitively applied to the same event location. Thus, block 630 may be used to map the number of triggering channels activated by the first threshold value for a given intensity.

For illustrative purposes, the following discussion of blocks 630-660 corresponds to determining a threshold value for a threshold circuit coupled to PMT 42 of FIG. 3. Accordingly, at block 630 a light source according to event 305 of FIG. 3 is applied to the detector. Subsequently, decision block 640 is processed.

At decision block 640, the triggering channels of the detector are examined to determine whether any of the triggering channels were engaged by the simulated event of block 630. If a trigger channel event signal is generated by either the right (PMTs 9, 27, and 42), middle (PMTs 51, 49, and 55), or left (PMTs 19, 36, and 48) channel, block 650 is processed. If none of the triggering channels are engaged, block 660 is processed.

At block 650, the number of engaged channels is determined. For one embodiment, the number of engaged channels are compared to the detector configuration identified in block 610. Accordingly, if the first threshold value is set to a low value, PMT 42 may result in the triggering of the left channel.

At block 660, the threshold value is re-set. In particular, the voltage value of Vref 525 is set to a second value. For one embodiment, if block 650 identified that the left channel was engaged by the simulated event of block 630, the second value may be set to a higher value than the first value. For another embodiment, if block 650 identified that the left channel was not engaged by the simulated event of block 630 or alternatively if none of the triggering channels were engaged, the second value may be set to a lower value than the first value. Subsequently, block 630 is reprocessed. Thus, as illustrated in flowchart 600, the loop of blocks 630-660 may be used to determine a threshold value that selectively transfers a PMT signal. For example, in response to event 305, the loop of blocks 630-660 may be used to select a threshold value that does not transfer the signals generated by PMT 42. For an alternative embodiment, the threshold value may be selected to ensure that for any given event location and intensity only two triggering channels are activated.

In the foregoing specification, the invention has been described with reference to specific exemplary embodiments thereof. It will, however, be evident that various modifications and changes may be made thereof without departing from the broader spirit and scope of the invention as set forth in the appended claims. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense. 

What is claimed is:
 1. An event detector system, comprising: a detector having a plurality of zones, each zone comprising a plurality of detector devices, wherein each detector device is operable to generate an event signal in response to an energy source; a plurality of adjustable threshold circuits coupled to the detector devices, the threshold circuits exhibiting thresholds adjusted as a function of the configuration of the zones of the detector devices.
 2. The system of claim 1, wherein the detector devices comprise photo multiplier tubes.
 3. The system of claim 2, wherein the energy source comprises a scintillation.
 4. The event detector system of claim 3, wherein the threshold circuits exhibit thresholds adjusted as a function of the configuration of the zones of detector devices and as a function of photomultiplier tube response.
 5. The event detector system of claim 3, wherein the threshold circuits exhibit thresholds adjusted as a function of the configuration of the zones of detector devices and as a function of photomultiplier tube response, wherein the detector exhibits a substantially uniform response to scintillation events.
 6. The event detector system of claim 3, wherein the threshold circuits exhibit thresholds adjusted as a function of the configuration of the zones of detector devices and as a function of photomultiplier tube response, wherein the detector exhibits a substantially uniform pixel sensitivity.
 7. The event detector system of claim 1, further comprising a trigger signal circuit coupled to the zones of detector devices, wherein the detector exhibits a substantially uniform trigger response.
 8. The event detector system of claim 1, further comprising: a plurality of trigger channels coupled to a plurality of the zones; and at least one detector device which is included in more than one zone, wherein an adjustable threshold circuit is adjusted to allow the at least one detector device to be active for only one trigger channel.
 9. The event detector system of claim 1, further comprising: a plurality of trigger channels coupled to a plurality of the zones; and at least one detector device which is included in more than one zone, wherein an adjustable threshold circuit is adjusted to allow the at least one detector device to be active for not more than two trigger channels.
 10. The event detector system of claim 1, further comprising: a plurality of shared detector devices which are included in more than one zone; and a plurality of unshared detector devices which are included in only one zone, wherein variable threshold circuits associated with the shared detector devices exhibit a different threshold characteristic than variable threshold circuits associated with the unshared detector devices.
 11. The event detector system of claim 10, wherein the variable threshold circuits associates with shared detector devices exhibit a higher threshold than the variable threshold circuits associated with unshared detector devices.
 12. A method for selecting a threshold value in an event detector, the method comprising: determining a detector configuration; applying a first threshold value to a threshold circuit; generating an energy source, wherein generating the energy source comprises directing a light source on the event detector; and examining the detector configuration to determine the number of triggering channels engaged by the energy source.
 13. The method of claim 12, further comprising: applying a second threshold value to the threshold circuit; and examining the detector configuration to determine the number of triggering channels engaged by the energy source.
 14. The method of claim 13, further comprising selecting the threshold value resulting in the lowest number of triggering channels engaged by the energy source.
 15. The method of claim 12, wherein determining the detector configuration comprises determining the photo multiplier tube locations of the event detector.
 16. The method of claim 12, wherein determining the detector configuration comprises determining the field of view of a crystal of the event detector.
 17. The method of claim 12, wherein determining the detector configuration comprises determining the photo multiplier tube assignment to the triggering channels of the event detector.
 18. A method for reducing the number of triggering channels engaged in an event detection, the method comprising: selecting a threshold value for a threshold circuit; and generating a plurality of event signal in response to an energy source, wherein each event signal is generated from a detector device; and clamping a first set of the plurality of event signals, wherein the clamping is operable to selectively transfer a second set of the plurality of event signals having a value greater than a threshold value.
 19. The method of claim 18, wherein the first set of the plurality of event signals are operable to engage a first triggering channel.
 20. The method of claim 19, wherein the second set of the plurality of event signals are operable to engage a second triggering channel.
 21. The method of claim 20, wherein the detector device comprises a photo multiplier tube.
 22. The method of claim 21, wherein the energy source comprises a scintillation. 